Is it safe to assume people make usernames just to sell for monetary gain?
Absolutely. But a general rule is that on popular platforms, every valuable name is taken already and likely has been taken since early after the very start of the platform.
They're referred to as "OG usernames", with OG not standing for Original as you might assume but instead for "Orginal Gangster"
How does one acquire valuable names then?
- By using programs like 'autoclaimers' that constantly scans a custom username list for availability and automatically registers it if it sees a username in the list is no longer taken. That can be because the owner changed or tried to swap it do a different account, or it could have been 'reset' as part of some platforms' policy to release usernames after an unspecified amount of inactivity. There is no fixed time limit to Instagram's reset policy for example, which is where the autoclaimer comes in.
-By illegal methods ranging anywhere from bruteforcing, 0day exploits, social engineering, extortion, SIM-swapping, fraudulent trademark applications, false obituaries, access to a platform's control panel functions via an inside employee, ....
Rich Arabs from the likes of Saudi Arabia, UAE, Dubai, ... are particularly obsessed with using IG, Twitter, Snapchat, ... usernames as a status symbol.
The ultimate status symbol username of course is having a single character/number/sign as your username. As I was once told by a wealthy Saudi during some other deal, the way they see it is that every Arab in their class of wealth can stunt with a bunch of supercars, huge houses, showing off tigers and lions, ... but when your username on Instagram is "G" (example), that's a truly unique status symbol for them.